1. Field of the Invention
The invention relates to an operating procedure for operating a recirculating-air sifter, particularly a cyclone circulating-air sifter for sifting cement, as well as a circulating-air sifter for carrying out the operating procedure.
2. Description of the Prior Art
In a recirculating-air sifter for sifting cement, the material to be sifted is customarily charged-in centrally from above and is distributed uniformly in a rising sifting-air stream by means of a spreading disc arranged in the shifting space. The coarse material from the sifting-air stream is precipitated into the housing part arranged underneath and finally leaves the sifter through a discharge stub at the lower part of the housing. The fine material in the sifting-air stream at the upper edge of the upper housing part is fed via an air discharge head to individual separating cyclones which are customarily arranged around the sifter in ring fashion. There, the separation of the fine material from the sifting air takes place, the sifting air being returned into the sifter via a manifold and a blower disposed on the outside. The uniform distribution of the sifting air over the sifting space entrance area is accomplished by means of guide vanes which are arranged in ring fashion in the central part of the housing. The separated fine material in the separator cyclones is discharged via pendulum flaps. A sifter described above is known, and disclosed for instance in German Pat. No. 1 178 284.
In the known recirculating-air sifters of the design described above, the amount of recirculating air must be reduced substantially and the speed of rotation of the scattering disc increased, when changing, for instance, from a coarser to a finer sifting product. This reduction of the amount of circulating air lead to deposits of material in the feed ducts from the sifter to the separating cyclone because the flow velocities are too low. In addition, the degree of separation in the separating cyclone is affected adversely by the lower supply of air. Due to the reduction of the amount of circulating air, a possible supply of fresh air for cooling the product must additionally be reduced so that an unfavorably high operating temperature results.